FIG. 1 is a block diagram of a subset of a full duplex wireless communication system 30, the subset having a subscriber unit ("SU") 10 and base stations ("BS") 22, 24, 26, and 28. Note: only a single SU 10 is shown for illustration of the invention, a complete wireless communication system normally would have many subscriber units 10 compared to the number of base stations 22, 24, 26, and 28 in the system. In the wireless communication system 30, the subscriber unit 10 transceives signals between the base stations 22, 24, 26, and 28. During a call, the subscriber unit 10 is linked to a single base station 22, 24, 26, and 28. Ideally, the subscriber unit 10 is linked to the base station 22, 24, 26, or 28 which provides the best transmission signal. For illustration purposes, subscriber unit 10 is linked to base station 22 in FIG. 1. As a consequence, subscriber unit 10 transceives signals with the base station 22.
The configuration shown in FIG. 1, however, is not static. The subscriber unit 10 may move in relation to base station 22 (and in some systems, the base stations may move) or interference or blockage may arise. Due to the changes in the radio propagation environment between the subscriber unit 10 and base station 22 this link may no longer provide the best transmission signal, in fact, the signal to noise ratio may not be high enough for clear transmission of signals from the subscriber unit 10 to the base station 22 and visa versa. In such a case, the subscriber unit 10 may change its link from base station 22 to another base station 24, 26, or 28 (depending which base station provides an adequate transmission signal and also has capacity to support a new call). The process of changing the base station to which the subscriber unit 10 is linked is termed a hand-off.
In many wireless communication systems, more particularly, modern personal communication systems ("PCS"), the subscriber unit has the task of deciding which base station has the strongest transmission signal. In some systems, the subscriber unit must also request a hand-off to that base station. Thus, in addition to transceiving signals from the base station to which the subscriber unit is currently linked, the subscriber unit must also sample signals from other base stations to determine which one can support adequate communication. Further, the subscriber unit 10 may also need to process control information to determine whether base stations which have adequate signal strength also have the capacity to transceive signals from the subscriber unit 10. Due to limitations imposed by the need to keep subscriber units small and low cost, it often is not possible for the subscriber unit to simultaneously maintain a call and perform tasks to initiate a hand-off. As a consequence, subscriber units may be precluded from starting a hand-off process until the link between the current base station has degraded below some criterion level. At this point, the communications is noticeably degraded or dangerously close to that point.
Ideally, the hand-off process should be "hit-less", i.e., the transceiving process in the subscriber unit 10 should not be interrupted during a hand-off nor should the communication link between the current base station degrade to an unacceptable level. The subscriber unit, however, has limited processing capabilities and, thus is not always able to evaluate received signals from nearby base stations or carry out two-way communications with other base stations while transceiving signals from its current base station. Thus, the transceiving process is usually interrupted during a hand-off for a noticeable time period (on the order of a second or more) in some PCS after the signal from the current base station has degraded to an unacceptable or even unusable level. This impact to the user of the subscriber unit 10 should be reduced or eliminated.
The subscriber unit, thus, needs a mechanism/process which enables the unit to monitor the signal quality of nearby base stations while transceiving signals from a current base station. Ideally, this mechanism/process should be active/performed continuously so when the signal quality from the current base station degrades below a threshold greater than the level where communication is interrupted, the subscriber unit will have a list of base stations with acceptable signal levels. The mechanism/process should not require changes to the existing architecture of the unit which would increase battery consumption or production costs of the subscriber unit 10.